Put it in perspective: You wouldn't want to have a baby until the timing was right, and remember, the grass is always greener on the other side. So many of the darling young mothers with darling husbands, houses, and babies, just wish they had a career of their own, less responsibility, and were able to get out of the house and have a pedicure. Consider the big decision when the time comes. There are potentially more involved than just yourself so give your baby its first gift and plan its future on more than just your needs.
Offer to babysit! No matter how cute the child, you will certainly be happy to hand him or her over to the parents at the end of the day. Remember, you have your whole life to be tied down by a baby. It's a full-time job.
Also, adopt a pet. (I realize I am comparing apples and oranges.) Basically, you are finding another outlet for your maternal instinct. :)
Take some time to examine what it is you are truly envying. Is it really kids you want? Or do you feel as if you need to "catch up" with your friends? Remember that their path might not necessarily be your path; it's okay if you don't have a child just to be on the same path as your friends. So if you do feel envious, think about where those feelings are really coming from.
Now if you really and truly want to have a child someday, keep in mind that what makes or breaks any experience is the timing. If it's not the right time, it will be a disaster for both you AND the child. Keep telling yourself that you want what's best for both of you and the best things are worth waiting for.
Try working with kids. Volunteer in an organization where you read to kids, help chaperone a field trip or even hold babies in a hospital. You need to see the childrearing thing from a few angles to be able to know if the time is right for you. When you around kids (not your own) a lot, you'll replace envy with admiration.